Answered

Why is the chlorine that is added to final,

treated waste water allowed to get used up

before it is discharged into a river or lake?

Verified Answer :

Hagrid

Chlorine is used as a disinfectant to wastewater treatment
plants. The addition of chlorine to the final treated water is because to make
sure that some of the microorganisms not killed during treatment is eliminated.
It is consumed before it is released to the environment to ensure that
microorganisms and other marine life are not killed in the process too.